The instant invention is concerned with improvements to a servo system of the type which may be used with a disk drive to cause the read/write head to be moved from track to track on the disk installed in the disk drive. This operation is usually called a "head seek" in parlance of the disk drive industry since the read/write head is caused to seek a new track in response to a "seek command". The disk installed in the disk drive can either be a permanently installed disk (in which case the disk is usually a "hard disk") or a removable disk (in which case the disk is most often a "floppy disk" although removable hard disks are known).
Prior art servo systems, when used with disk drives, typically have two modes of operation: a velocity mode and a position mode. The servo is in its velocity mode when a head seek operation is underway and is in its position mode when the head is merely trying to accurately follow a given track around the disk (the tracks are normally concentric). The servo system disclosed by this patent also has these two modes of operation. The velocity mode is implemented by among other things, an appropriately programmed microprocessor, and therefore it is a "pure" digital implementation, whereas prior art devices used analog implementations. The position mode is implemented by analog circuitry, although in some embodiments the microprocessor is programmed to assist the analog circuitry in keeping the head accurately following a track.